Theremin. It is often claimed that it is the only instrument you can play without touching it. That's not actually true. Excluding the human voice as an argument of semantics, the theremin is just the only one that is well-known. Early electronic instruments are most often bulky, clunky and difficult to use. All of these were tube-driven and had the downsides of being both hot and fragile. The
Theremin was pretty elegant compared to the others. Most people are at least passingly familiar with early keyboard-based devices. But there were many others more like the Marimbalite— a several hundred pound device containing more than 60 photo-electric tubes...early optical sensors requiring no direct contact to play. Steam-punk genius yes, but practical instrument, no.
I've found a number of these luthier's-nightmare devices in back issues of radio and electronics magazines: the Trautonium, Telepiano, Pianotron, the Syntonic Organ, The Polytone, the Electo-musical Trombone, the Radio Organ and the Radio Violin... Let's pause on those last two.
The Radio Organ contained no radio. Much like the Marimbalite, it worked mostly from a series of photo-electric cells. But in 1934 a lot of gadgets had the word 'radio' tacked on the front of them for marketing reasons. In 1934 James Nuttall and Fred Sammis built a "radio organ" from an old keyboard, tin cans, a loud speaker, a dishwasher motor, and photo-electic cells. The organ contains a film-track disc of recorded tones. Light passes through a shutter through disc and to strike the photocells and a tone is generated.There is little hardware on board you'd mistake for a radio.
The Radio Violin is a bit more elusive. There were two different "Radio Violins" written up in the 1930s. The first was in Radio Craft magazine December of 1931. The article was short but the opening sentence made it sound like the amalgam of violins and radios was some long heralded event: "Radio programs may now be received through the medium of the violin." The small diagram that followed described the soldering and attaching of a earphone earpiece to the violin so that it's sound may resonate int he violin body. The violinist may then accompany the radio live.
But there was another radio violin that debuted in August of 1934. It was written up by Modern Mechanix and Popular Science. That Radio Violin had no soundboard and consisted of only the exterior violin-shaped frame. the vibrations of the strings were picked up by a magnetic pickup much like a modern electric guitar. From the violin a cable then connected that signal to an amplifier and loud speaker. No radios were harmed in the making of this device.